Charge pumps are used to generate a higher voltage than the supply voltages available for an application. For example, in a Liquid Crystal Display (LCD) driver, a charge pump is used to generate the bias voltages required by the driving schemes.
In the LCD driving integrated circuits (ICs), the trend is nowadays for manufacturers to develop ever higher resolution and color displays for mobile terminals (phones, PDAs, etc.). These displays need a large current at a voltage approaching 20V, which has to be generated by the LCD driving IC. Charge pumps are used, with on-chip capacitors or with external capacitors, in order to achieve this voltage.
A characteristic of modern driving schemes for passive liquid crystal displays (LCDs) is that several DC voltage levels are switched to supply the rows and columns of a matrix of pixels (assimilated with capacitors). LCD driving ICs implementing these driving schemes have, due to the switching, a current consumption that is not negligible.
Every voltage level of (for example 7 different voltages) necessary to drive an LCD using the Multiple Row Addressing (MRA) scheme (with 4 rows selected at a time) would see a different current consumption. Depending on the pattern displayed on the LCD panel, the loads of the different voltage levels can become very unbalanced (sometimes with a factor of 10). Because of this highly random load distribution, it is very difficult to develop a bias level generator which is optimal. With a capacitive voltage booster for instance, it is known that the higher a voltage is needed, the higher would be the current consumption.
Recent developments propose a solution that is not yet optimal. This solution is based on pumping in a first step to a pump output level that is about half of the highest voltage, supplying the voltage levels which are lower than this pump output level, then using a voltage doubler to supply the rest of the higher voltage levels.
A charge pump with several charge stages being arranged in series are disclosed in the published US patent application US 2002/0114199 A1. A required multiplication factor is adjustable by activating/deactivating a number of stages.
The known approaches for providing a set of voltage levels are not satisfying if the load is unbalanced, for instance.
It is thus an object of the present invention to provide a solution that takes into account the characteristics of the current load
It is a further objective of the present invention to improve conventional charge pumps and to provide charge pumps having a reduced current consumption.
It is a further objective of the present invention to improve conventional LCD drivers and to reduce their current consumption.